Oxy-Acetylene Weld Overlay Repair of SAG Mill Sliding Bearings
1. Definition and Technical Principles
Oxy-acetylene weld overlay repair is a flame-based surface engineering technique that utilizes the combustion of acetylene (C₂H₂) and oxygen (O₂) to generate a localized high-temperature flame (approximately 3,100–3,300 °C) for depositing a protective or restorative alloy layer onto damaged bearing surfaces. When applied to SAG (Semi-Autogenous Grinding) mill sliding bearings, this method restores dimensional accuracy, surface hardness, and load-bearing capacity of worn or galled bearing pads, trunnion seats, and thrust collars without requiring removal of the entire bearing assembly.
The fundamental principle involves creating a carburizing or neutral flame that melts a thin layer of the base metal while simultaneously depositing a filler alloy (typically nickel-based, cobalt-based, or iron-based hardfacing wire) onto the prepared surface. The controlled heat input allows for localized repair in confined spaces where arc welding equipment cannot be practically deployed, making it uniquely suited for in-situ repair of large rotating equipment bearings in mining and mineral processing operations.
1.1 Thermodynamic Considerations for Bearing Repair
The heat input in oxy-acetylene welding is significantly lower and more diffuse than in arc welding processes. For SAG mill sliding bearing repair, this characteristic is both advantageous and challenging:
- Advantage: Reduced thermal distortion of precision-machined bearing surfaces, critical for maintaining the bearing clearance and alignment tolerances required in SAG mill operation (typically 0.05–0.15 mm per 100 mm diameter)
- Challenge: Lower deposition rates (approximately 0.5–1.5 kg/h) necessitate extended repair durations for heavily worn surfaces
- Heat Affected Zone (HAZ): Typically 3–8 mm wide, requiring careful management to avoid altering the microstructure of the base bearing bronze or steel substrate
2. Category and Business Positioning
This repair technique falls within the company's Weld Overlay and Surface Restoration business segment, specifically in the category of in-situ field repair and emergency maintenance services. It represents the company's capability to provide rapid, on-site restoration of critical mining equipment components where:
- Equipment downtime costs exceed USD 10,000–50,000 per hour
- Logistics constraints prevent component removal and off-site repair
- The bearing assembly is integrated into a structure that cannot be disassembled without major capital expenditure
Within the company's three primary technology routes, oxy-acetylene weld overlay repair functions as a complementary field service capability that bridges the gap between permanent cladding solutions (TIG/MIG weld overlay, hydraulic explosive bonding, explosion welding) and the immediate operational needs of mining customers. It demonstrates the company's versatility in addressing the full lifecycle of bearing protection — from new cladding installation through to field repair and restoration.
3. Technical Purpose and Value
3.1 Primary Technical Objectives
The repair of SAG mill sliding bearings via oxy-acetylene weld overlay serves several critical technical purposes:
- Dimensional Restoration: Rebuild worn bearing surfaces to original manufacturer specifications, typically restoring 1–15 mm of material loss on bearing pads, trunnion seats, and thrust collar surfaces
- Surface Enhancement: Deposit hardfacing alloys (e.g., Stellite 6, Incoloy 800, or Ni-Cr-Mo iron-based alloys) to improve wear resistance, galling resistance, and load distribution characteristics beyond the original base material properties
- Corrosion Resistance: Protect against mineral slurry ingress and chemical attack from process fluids that accumulate in bearing housings
- Life Extension: Extend bearing service intervals from typical 6–12 month replacement cycles to 24–36 months, reducing total cost of ownership by 40–60%
3.2 Customer Value Proposition
For mining operations, this repair capability delivers quantifiable value through:
- Unplanned downtime reduction: Field repair eliminates 7–14 day logistics delays associated with shipping large bearing assemblies to off-site workshops
- Capital avoidance: Prevents replacement of entire bearing assemblies valued at USD 50,000–200,000 per set
- Production continuity: Enables bearing repair during scheduled maintenance windows (typically 2–5 days) rather than extended shutdowns
- Performance improvement: Hardfacing overlay often exceeds original bearing material properties, providing enhanced performance in subsequent service cycles
4. Key Process and Implementation Points
4.1 Pre-Repair Assessment and Preparation
Successful oxy-acetylene weld overlay repair of SAG mill sliding bearings requires rigorous pre-repair assessment:
| Assessment Parameter | Acceptance Criteria | Measurement Method |
|---|---|---|
| Surface Wear Depth | ≤ 20% of bearing pad thickness | Ultrasonic thickness gauge / profile comparison |
| Base Material Identification | Confirmed alloy composition | Optical emission spectrometry / chemical analysis |
| Crack Inspection | No cracks deeper than 0.5 mm | Penetrant testing (PT) per ASTM E709 |
| Bearing Clearance | Within OEM ±0.05 mm specification | Feeler gauges / dial indicator measurement |
| Surface Contamination | Free of oil, grease, mineral slurry | Visual inspection + solvent cleaning verification |
4.2 Surface Preparation Protocol
Surface preparation is the single most critical factor in ensuring weld overlay adhesion and long-term service integrity:
- Complete disassembly: Remove bearing pads, thrust collars, and associated components; document original dimensions and alignment
- Chemical degreasing: Apply solvent-based cleaner to remove all lubricating oil, hydraulic fluid, and mineral residue
- Mechanical grinding: Grind worn surfaces with coarse (36–60 grit) followed by fine (120–180 grit) abrasive to expose clean base metal
- Heat treatment assessment: Determine if pre-heat is required based on base material carbon equivalent (CE ≥ 0.4% requires pre-heat of 150–250 °C)
- Flux application: Apply appropriate flux (e.g., FNX-1 for nickel-based alloys, FNX-4 for cobalt-based alloys) to prevent oxide inclusion
4.3 Weld Overlay Execution Parameters
| Parameter | Typical Specification | Rationale |
|---|---|---|
| Flame Type | Slightly carburizing (C/O ratio 1.05–1.10) | Provides adequate heat input while minimizing oxide formation |
| Flame Length | 10–15 mm from nozzle to workpiece | Optimizes heat concentration and penetration control |
| Welding Speed | 30–60 mm/min (dependent on wire diameter) | Controls bead profile and HAZ width |
| Wire Diameter | 2.0–3.0 mm (typical hardfacing wire) | Balances deposition rate with bead control |
| Interpass Temperature | ≤ 150 °C (nickel alloys) / ≤ 200 °C (iron alloys) | Prevents grain coarsening and cracking |
| Pre-heat Temperature | 150–350 °C (material dependent) | Reduces thermal stress and hydrogen cracking risk |
| Post-Weld Heat Treatment | 500–650 °C × 2–4 hours (if required) | Relieves residual stress and optimizes microstructure |
| Maximum Single Pass Thickness | 3–5 mm | Prevents cracking and ensures proper fusion |
4.4 Multi-Pass Build-Up Strategy
For bearing surfaces with significant material loss (>3 mm), a multi-pass build-up strategy is employed:
- Pass 1 (Bonding pass): Thin layer (1–2 mm) using transition alloy to ensure metallurgical compatibility between base material and hardfacing
- Passes 2–N (Build-up passes): Hardfacing alloy deposited in 2–4 mm increments with interpass grinding to ensure sound bonding
- Final Pass (Surface pass): Optimized for surface finish (Ra ≤ 3.2 μm) and dimensional accuracy
4.5 Post-Weld Finishing
After weld overlay completion, the bearing surface undergoes:
- Machining to final dimensional tolerance (typically ±0.02 mm for bearing pads, ±0.01 mm for thrust collars)
- Surface grinding to achieve specified roughness (Ra 0.8–1.6 μm for sliding contact surfaces)
- Hardness verification (target: 40–55 HRC for Stellite overlay; 35–45 HRC for Ni-based overlay)
- Final dimensional inspection using CMM or precision gauges
- Reassembly with new lubricant and alignment verification
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
The oxy-acetylene weld overlay repair of SAG mill sliding bearings shall comply with the following standards framework:
| Standard | Scope of Application |
|---|---|
| ASTM A396 / A396M | Carbon and alloy steel bearing steel for sliding bearings |
| ASTM E709 | Nondestructive examination by penetrant testing |
| ASTM E165 | Nondestructive examination by magnetic particle testing |
| ASTM A276 | Castings, carbon and alloy steel, for general application |
| ASME BPVC Section V | NDE acceptance criteria for weld repair |
| ASME BPVC Section IX | Welder qualification and WPS requirements |
| ISO 9093 | Welding — Surface repair welding by arc processes |
| ISO 1091 | Welding — Classification of arc-welding processes |
| GB/T 11345 | Ultrasonic testing of welds — acceptance levels |
| NACE MR0175 / ISO 15156 | Materials for H₂S-containing environments (where applicable) |
| API 660 | Horizontal and vertical process pumps (bearing design reference) |
| ISO 1328 | Industrial gears — tolerances (for associated gear/coupling alignment) |
5.2 Acceptance Criteria
Completed weld overlay repairs shall meet the following acceptance criteria:
- Visual Inspection (VT): No cracks, porosity, undercut, or excessive spatter; smooth, uniform bead profile
- Penetrant Testing (PT): No linear indications exceeding 3 mm in length or 0.1 mm in width (per ASME Section V, Article 7)
- Magnetic Particle Testing (MT): No indications of cracking, lack of fusion, or slag inclusion (per ASTM E165, Method A or B)
- Ultrasonic Testing (UT): No volumetric indications exceeding acceptance Level II per GB/T 11345 or ASME Section V Article 4
- Hardness Test: Overlay hardness within specified range (±5 HRC of target); HAZ hardness not exceeding 350 HV for steel substrates
- Dimensional Verification: Final machined surface within OEM tolerance specifications
- Chemical Analysis: Overlay alloy composition within specification (ASTM A567 for Stellite, ASTM B424 for Ni-based alloys)
6. Common Risks and Controls
6.1 Technical Risk Matrix
| Risk Category | Risk Description | Likelihood | Impact | Control Measures |
|---|---|---|---|---|
| Cracking | Hot cracking in overlay due to high sulfur/phosphorus in base metal | Medium | High | Pre-heat control; low-sulfur filler selection; interpass temperature monitoring |
| Porosity | Gas porosity from inadequate flux or contaminated surface | Medium | Medium | Thorough cleaning; proper flux coverage; controlled atmosphere |
| Insufficient Penetration | Poor fusion between base metal and overlay | Low | Critical | Adequate pre-heat; proper flame adjustment; verified welder qualification |
| Thermal Distortion | Dimensional change exceeding machining allowance | Medium | High | Staggered welding sequence; controlled heat input; post-weld stress relief |
| Hydrogen Cracking | Delayed cracking in HAZ of high-strength bearing steel | Low | Critical | Low-hydrogen filler; post-weld bake (200 °C × 4 hours); avoid high CE base materials |
| Operational Safety | Acetylene explosion or oxygen burns during field operations | Low | Critical | Gas cylinder management per OSHA 29 CFR 1910.253; proper PPE; fire watch |
| Equipment Re-alignment | Bearing repair introduces misalignment in mill drive train | Medium | High | Laser alignment verification post-repair; documented pre/post dimensions |
6.2 Critical Control Points
The following critical control points (CCPs) must be verified at each stage:
- CCP-1: Base Metal Verification — Confirmed composition matches repair WPS; carbon equivalent calculated and pre-heat requirement determined
- CCP-2: Surface Preparation — Clean, oxide-free surface verified by visual inspection; roughness profile measured (Ra 12.5–25 μm for mechanical bonding)
- CCP-3: Welder Qualification — Active qualification per ASME Section IX or equivalent; demonstrated proficiency in oxy-acetylene hardfacing on similar materials
- CCP-4: Interpass Monitoring — Temperature logged at each pass; deviations >50 °C from specification trigger stop-work and re-evaluation
- CCP-5: Post-Weld NDE — All required NDE completed and documented before machining; any indication requiring rework addressed before proceeding
- CCP-6: Final Dimensional Verification — CMM or precision gauge verification against OEM drawing; certificate of conformity issued
7. Application Scenarios Across Company Technology Routes
7.1 Integration with TIG/MIG Weld Overlay Route
Oxy-acetylene weld overlay repair complements the company's primary TIG/MIG weld overlay capability in the following scenarios:
- Pre-qualification demonstration: Successful field repairs using oxy-acetylene techniques provide performance data that supports WPS qualification for TIG/MIG overlay procedures on similar bearing applications
- Hybrid repair strategy: For heavily worn bearings, oxy-acetylene is used for initial bulk build-up (high deposition rate in confined spaces), followed by TIG finishing for precision surface preparation and final dimensional machining
- Transition layer deposition: When overlaying dissimilar materials (e.g., nickel-based hardfacing on carbon steel bearing housings), oxy-acetylene can be used for the transition pass while TIG handles subsequent layers for superior quality
- Emergency repair bridge: When TIG equipment is unavailable at remote mine sites, oxy-acetylene provides a viable interim repair solution that maintains equipment availability
7.2 Integration with Hydraulic Explosive Bonding Route
The hydraulic explosive bonding (HEB) route primarily addresses permanent cladding of large structural components. Oxy-acetylene bearing repair contributes to this route through:
- Post-bonding repair capability: When HEB-clad components suffer localized damage (impact, corrosion pitting), oxy-acetylene provides targeted repair without disturbing the bonded interface
- Edge sealing: After hydraulic explosive bonding of bearing housings or trunnion supports, oxy-acetylene weld overlay can seal perimeter edges and address any bonding discontinuities
- Dimensional correction: HEB processes may introduce minor dimensional deviations; oxy-acetylene build-up followed by machining restores precise bearing geometry
7.3 Integration with Explosion Welding Route
Explosion welding produces high-integrity metallurgical bonds for permanent cladding. The oxy-acetylene repair capability supports this route by:
- Field maintenance of explosion-welded components: Explosion-welded bearing assemblies in service may require periodic surface restoration; oxy-acetylene provides on-site repair without dismantling
- Prototype qualification: Oxy-acetylene repair of legacy bearing designs generates metallurgical and performance data that informs explosion welding design parameters for next-generation components
- Customer relationship development: Field repair services establish trust and technical credibility that leads to specification of explosion welding for new equipment procurement
8. Qualification Building and Quality Management
8.1 Welder Qualification Requirements
Personnel performing oxy-acetylene weld overlay repair of SAG mill sliding bearings must maintain active qualification:
- Qualified per ASME Section IX, QW-301 through QW-312 (flame welding qualifications)
- Minimum 500 hours documented experience in hardfacing/weld overlay on ferrous and non-ferrous substrates
- Demonstrated competency in oxy-acetylene hardfacing on bearing steels (ASTM A396), cast iron, and bronze alloys
- Annual requalification with hardness testing, macrographic examination, and PT of qualification coupon
- Documented training in SAG mill bearing geometry, alignment tolerances, and operational requirements
8.2 Procedure Qualification (WPS/PQR)
Each repair application requires a qualified Welding Procedure Specification (WPS) supported by a Procedure Qualification Record (PQR):
| WPS Element | Required Specification |
|---|---|
| Base Material Group | Per ASME Section IX, Group 1.1 (carbon steel) or Group 1.4 (cast iron) |
| Filler Metal | Per AWS A5.15 (Stellite), AWS A5.22 (Ni-based), or AWS A5.23 (Fe-based) |
| Flame Type | Carburizing, neutral, or oxidizing — specified by WPS |
| Heat Input Range | 0.5–2.0 kJ/mm (flame welding equivalent) |
| Pre-heat / Interpass | Material-specific temperatures with monitoring requirements |
| Post-Weld Treatment | Stress relief parameters or as-received (with justification) |
| Essential Variables | Per ASME Section IX, QW-301 (flame welding) |
8.3 Documentation and Traceability
Complete traceability documentation for each repair includes:
- Repair authorization and customer work order
- As-found condition report with photographs and dimensional measurements
- WPS reference number and welder qualification ID
- Material certificates for filler metal and flux (mill certificates with batch traceability)
- Process parameter log (pre-heat, interpass temperatures, flame settings, pass sequence)
- NDE reports (VT, PT, MT, UT as applicable) with Level II or III certification
- Post-repair dimensional certificate and hardness report
- Final reassembly alignment report
- Repair completion certificate signed by authorized quality representative
9. Conclusion and Strategic Significance
The oxy-acetylene weld overlay repair of SAG mill sliding bearings represents a high-value, technically demanding capability that positions Cladding Technology Shanxi Co., Ltd. as a comprehensive surface engineering and repair service provider. This capability:
- Expands market reach into the mining and mineral processing sector, where SAG mill bearing failures are among the most costly and frequent equipment failures
- Builds qualification foundation for more complex TIG/MIG overlay and bonding applications by demonstrating metallurgical understanding and process control
- Generates customer trust through successful field repairs, creating entry points for permanent cladding solutions via all three technology routes
- Supports quality management systems (ISO 9001, ISO 3834) by maintaining documented procedures, qualified personnel, and traceable repair records
As the company scales its operations, the oxy-acetylene repair capability serves as both a revenue-generating service line and a strategic platform for technology development, customer engagement, and qualification portfolio expansion across the full spectrum of weld overlay and cladding applications.